Toward the Sustainable Synthesis of Propanols from Renewable Glycerol Over Moo3-Al2o3 Supported Palladium Catalysts

catalysts Article Toward the Sustainable Synthesis of Propanols from Renewable Glycerol over MoO3-Al2O3 Supported Palladium Catalysts Shanthi Priya Samudrala * and Sankar Bhattacharya Department of Chemical Engineering, Faculty of Engineering, Monash University, Melbourne 3800, Australia; [email protected] * Correspondence: [email protected]; Tel.: +61-3-9905-8162 Received: 17 August 2018; Accepted: 6 September 2018; Published: 9 September 2018 Abstract: The catalytic conversion of glycerol to value-added propanols is a promising synthetic route that holds the potential to overcome the glycerol oversupply from the biodiesel industry. In this study, selective hydrogenolysis of 10 wt% aqueous bio-glycerol to 1-propanol and 2-propanol was performed in the vapor phase, fixed-bed reactor by using environmentally friendly bifunctional Pd/MoO3-Al2O3 catalysts prepared by wetness impregnation method. The physicochemical properties of these catalysts were derived from various techniques such as X-ray diffraction, 27 NH3-temperature programmed desorption, scanning electron microscopy, Al NMR spectroscopy, surface area analysis, and thermogravimetric analysis. The catalytic activity results depicted that a high catalytic activity (>80%) with very high selectivity (>90%) to 1-propanol and 2-propanol was obtained over all the catalysts evaluated in a continuously fed, fixed-bed reactor. However, among all others, 2 wt% Pd/MoO3-Al2O3 catalyst was the most active and selective to propanols. The synergic interaction between the palladium and MoO3 on Al2O3 support and high strength weak to moderate acid sites of the catalyst were solely responsible for the high catalytic activity. The maximum glycerol conversion of 88.4% with 91.3% selectivity to propanols was achieved at an optimum reaction condition of 210 ◦C and 1 bar pressure after 3 h of glycerol hydrogenolysis reaction. Keywords: hydrogenolysis; glycerol; 1-propanol; 2-propanol; palladium catalyst 1. Introduction Alternate sustainable energy resources are vital because of dwindling petroleum reserves and mounting environmental alarms that are allied with fossil fuel exploitation. As a result, alternative bio-based fuels have emerged as the long-standing solution as they are renewable and carbon dioxide neutral [1]. Biodiesel is one such alternative fuel which has received much attention with both demand and production tremendously increased over the last few years. However, along with biodiesel, also known as alkyl esters of long chain fatty acids (C14–C24), a great deal of glycerol as a by-product is also generated, typically equaling 10% of the whole production volume. The crude glycerol, if not handled properly, ends up as a waste product that has low value and is costly to purify in addition to jeopardizing the environmentally friendly nature of the whole biodiesel production process [2]. Valorization of glycerol is, therefore, necessary to enhance the sustainability of the biodiesel industry. Glycerol, the simplest tri-hydroxy alcohol, is a highly versatile product, with many potential applications. The biodegradability and multi-functional nature of glycerol makes it a promising precursor for the production of high-value bio-renewable fuel/chemical products through various processes involving heterogeneous catalysis, e.g., acetalization [3,4], esterification [5], etherification [6], oxidation [7], dehydration [8], hydrogenolysis [9], and catalytic reforming [10]. The glycerol-derived Catalysts 2018, 8, 385; doi:10.3390/catal8090385 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 385 2 of 18 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 fuelprocesses and chemical involving products heterogeneous include catalysis, liquid/gaseous e.g., acetalization fuels, [3,4], fuel esterification additives, and[5], etherification chemicals such as solketal,[6], oxidation glycerol [7], mono-esters,dehydration [8], glyceric hydrogenolysis acid, 1,3-dihydroxyacetone, [9], and catalytic reforming epichlorohydrin, [10]. The glycerol- glycidol, tartronicderived acid, fuel lacticand chemical acid, acrylonitrile,products include 1,2-propanediol, liquid/gaseous fuels, and fuel 1,3-propanediol, additives, and chemicals etc. such Catalytic hydrogenolysisas solketal, glycerol of glycerol mono-esters, is a promising glyceric acid, approach, 1,3-dihydroxyacetone, resulting in the epichlorohydrin, formation of glycidol, commodity chemicalstartronic such acid, as 1,2-propanediol,lactic acid, acrylonitrile, 1,3-propanediol, 1,2-propanediol, ethylene and glycol, 1,3-propanediol, propanols etc. (1-propanol Catalytic and 2-propanol),hydrogenolysis lower alcohols,of glycerol and is a hydrocarbons promising approach, [11]. Significant resulting in efforts the formation have been of made commodity to convert glycerolchemicals into propanediols, such as 1,2-propanediol, but direct 1,3-propanediol, production of propanolsethylene glycol, via glycerol propanols hydrogenolysis (1-propanol and received 2- propanol), lower alcohols, and hydrocarbons [11]. Significant efforts have been made to convert limited attention. glycerol into propanediols, but direct production of propanols via glycerol hydrogenolysis received 1-propanol (1-PrOH) and 2-propanol (2-PrOH) are valuable commodity chemicals conventionally limited attention. produced1-propanol from fossil-based (1-PrOH) feedstocks and 2-propanol ethylene and(2-PrOH) propylene are byvaluable hydroformylation-hydrogenation commodity chemicals and hydrationconventionally reaction produced processes, from fossil-based respectively feedstocks [12]. 1-PrOH ethylene has and potential propylene applications, by hydroformylation- primarily as a solventhydrogenation in the pharmaceutical, and hydration paint, reaction cosmetics, processes, and cellulose respectively ester industries, [12]. 1-PrOH organic has intermediate potential for the synthesisapplications, of important primarily chemicalas a solvent commodities, in the pharmaceutical, and considered paint, ascosmetics, the next-generation and cellulose gasoline ester to petroleumindustries, substitute. organic 2-PrOHintermediate is extensively for the synthesis used of as important an industrial chemical solvent commodities, and disinfectant, and considered with major applicationsas the next-generation in the pharmaceutical gasoline to industrypetroleum and substitute. auto industry 2-PrOH is [13 extensively]. Due to theused growing as an industrial demand of thesesolvent commodities, and disinfectant, the production with major of applications 1-PrOH and in 2-PrOHthe pharmaceutical based on bio-basedindustry and glycerol auto industry (Scheme 1) [13]. Due to the growing demand of these commodities, the production of 1-PrOH and 2-PrOH based appears to be an attractive approach regarding sustainability and energy efficiency compared to the on bio-based glycerol (Scheme 1) appears to be an attractive approach regarding sustainability and process based on petroleum-derived feedstocks. energy efficiency compared to the process based on petroleum-derived feedstocks. SchemeScheme 1. 1.Synthetic Syntheticroutes routes to 1-propanol and and 2-propanol. 2-propanol. HydrogenolysisHydrogenolysis of glycerolof glycerol generally generally takestakes place via via the the dehydration-hydrogenation dehydration-hydrogenation pathway pathway overover metal-acid metal-acid bifunctional bifunctional catalystscatalysts [8,14–16]. [8,14–16 Noble]. Noble metal-based metal-based catalysts catalysts have proven have to provenbe highly to be effective for glycerol hydrogenolysis [17]. Very few catalytic systems based on Ni [18], Ir [19], Rh highly effective for glycerol hydrogenolysis [17]. Very few catalytic systems based on Ni [18], Ir [19], [20,21], Pt [22,23], and Ru [24,25] based catalysts have so far been reported for the direct synthesis of Rh [20,21], Pt [22,23], and Ru [24,25] based catalysts have so far been reported for the direct synthesis biopropanols from glycerol, both in batch and fixed bed reactors. Previous research by Zhu et al. [26] of biopropanols from glycerol, both in batch and fixed bed reactors. Previous research by Zhu et al. [26] on liquid phase glycerol hydrogenolysis to 1-propanol and 2-propanol over Pt-H4SiW12O40/ZrO2 on liquidbifunctional phase catalysts glycerol at hydrogenolysis 200 °C and 5 MPa to H 1-propanol2 pressure revealed and 2-propanol that appropriate over Pt-Hmetal-acid4SiW 12balanceO40/ZrO 2 ◦ bifunctionalis important catalysts to promote at 200 theC anddouble 5 MPa dehydration-hydrogenation H2 pressure revealed thatability appropriate of glycerol. metal-acid In our earlier balance is importantstudy [27], to we promote have explored the double the effect dehydration-hydrogenation of different heteropolyacids abilityon the glycerol of glycerol. conversion In our and earlier studyselectivity [27], we to have propanols explored by using the effect a Pt-HPA/ZrO of different2 catalytic heteropolyacids system and ona continuous the glycerol flow, conversion fixed-bed and reactor set-up under ambient pressure. It was found that the metal dispersion and acidity of the selectivity to propanols by using a Pt-HPA/ZrO2 catalytic system and a continuous flow, fixed-bed reactorcatalyst set-up attributed under ambientto the high pressure. activity. Lin It was et al. found [28] reported that the the metal combined dispersion use of zeolite and acidity and Ni- of the catalyst attributed to the high activity. Lin et al. [28] reported the combined use of zeolite and Ni-based catalysts as a sequential

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